Ceramic coated ferrous article



Patented Aug. 27, 1946 I 2,406,534 CERAMIC COATED FERROUS ARTICLE Luther D. Fetterolf, Palmerton, Pa., assignor to C The New Jersey Zinc Company, New York, N. Y.,

I a corporation of New Jersey Application March 10, 1944, Serial No. 525,972

4 Claims.

This invention relates to the protection of iron and steel against oxidation when subjected to high temperatures such as 1000 to 1300 C., and has for its object the provision of a protective ceramic coating on iron and steel, and more particularly on externally heated retorts made of iron or steel. The invention is particularly applicable to iron or steel retorts in the production of magnesium by pyrometallurgical reduction or smelting, and that application of the invention is herein emphasized, but without thereby limiting the scope of the invention.

Magnesium reduction or smelting is customarily carried out in externally heated metal retorts, usually alloy steel tubes or cylinders havin an internal diameter of about 10 inches and a wall thickness of about 1 /8 inches. The retorts are now customarily made of a steel alloy containing 25% chromium and 15% nickel. The retorts are mounted in a furnace structure with feet or more of their length in the heated zone. The reducing charge consists of a briquetted mixture of calcined dolomite (or equivalent magnesiferous material) and ferrosilicon or equivalent reducing agent). The operation is intermittent or batch, and the operating cycle comprises (l) charging the briquets to the retorts, (2) connecting the retorts to a vacuum line capable of maintaining within the retorts a vacuum of 200-250 microns of mercury during the initial stage of the reduction and of 50-100m'i'crons of mercury during the final stage of the reduction,

(3) heating the charge' to a temperature of 1150-1250 C. for 7 to 8 hours, (4) removing the condensed magnesium vapor from the cold extension of th'e'retort, and (5) discharging the Worked-off briquets or spent residue from the retorts. The metal retorts are thu subjected to atmospheric pressure on the outside and to a relatively high vacuum on the inside during the reducing stage of the operating cycle. This results in a considerable compressive stress on the retort wall. The retort ultimately fail by gradually collapsing under this lon continued comof the iron or steel retort is provided with a ceramic coating that effectively protects the iron or steel fromoxidation and corrosion by the hot heating gases. Steel retorts so protected have much longer life than unprotected steel retorts. The invention will be best understood from the following description taken in conjunction with the single figure of the accompanying drawing illustrating in section an alloy steel retort provided with the composite ceramic coating of the invention.

In its broad aspect the invention involves protecting ferrous articles, that is articles made of iron or steel, against oxidation at elevated-temperatures (e. g. 1000 to 1300 C.) by a composite ceramic coating comprising a base layer of rea fractory material nonreactive with iron oxide and a covering layer of a vitreous material firmly adhering to the base layer and forming an impervious film thereover. It is now my preferred practice to use a magnesiferous refractory, such as magnesite or periclase, as the base layer, other suitable refractory materials for the base layer include chromite-magnesite, chrome, forsterite, alumina and the like. Feldspar is admirably adapted as the vitreous material of the covering layer. Feldspar fuses to a glass attemperaturesaround 1150-1250 C. If the operatingtemperature to which the ferrous article is to be subjected is substantially lower than 1150 (3., a fiux can be added to the feldspar to lower its fusion temperature correspondingly. Ceramic pervious film at the contemplated elevated oper-:

ating temperature.

In practicing the invention, the surface of the ferrous article to be protected, e. g. the exterior.

surface of a steel magnesium reducing retort, is covered with a relatively thick (e. g. 3 5 to inch) base layer of the refractory material, and the base coat is then covered by a relatively thin cover coat of a vitreous material adapted to fuse to form an impervious fihn at the elevated operating temperature to which the ferrous article is to be subjected. The covering layer of vitreous material firmly adheres to the base coat and forms an impervious film thereover. Preferably, the vitreous material slightly penetrates the base layer to form a vitreous bond therewith.

The base layer of refractory material may be applied to the ferrou article in any appropriate manner.

Thus, for example, the refractory material may be made into a slurry with water, and

theslurry appliedto the surface to be protected by dipping, spraying, or the like. The base layer may be dried before'applying the covering layer, or the covering layer may be applied to the still wet base layer. The covering layer may be applied to the base layer in an appropriate manner,'but when'applied to the still wet base layer it is advantageous to appl the covering layer by spraying a slurr of the vitreous material and water.

penetrate through the base layer andreact with the ferrous article. The layers should be slowly 7 dried and fired t prevent disintegration by too The firing may be ef; fectedby exposing the dried'coatingto'the e1e-" vated temperature of its contemplatedplaceof rapid expulsion of water.

use, as for example in the case of an ordinarysteel magnesium reducing retort by installingthe retort, 'coated as herein described, in the magnesium reduction furn-ace. However efiected, firin fuses or melts the covering layer and covers or encloses the refractory layer with an impervious vitreous film,

Byway of example, the coating of the ferrous article with a base layer of magnesiteor periclase and a covering layer of feldspar will be particu. larly described as follows :fThe magnesite or periclaseimade up in slurry form bymilling with Water and a small amount of clay in order to maintain the magnesia particles in water suspension About 5% of clay generally needed,

though this depends upon th suspensive properties-of the clay' The clay has the further func tion of contributing strength to the dried and fired coating. Magnesia grain, whether in the form Ofm-agnesite or periclase, has no bonding proper'tie itself and is to refractory to develop any fired strength at a temperature as low as 1200 C.

' --'-Ihe consistency of the slurry is adjusted, by the addition of water, to the desired method of ap plying'thelayereither spraying or dipping. It may be desirable to adjust the yield value of the slurry by adding some electrolyte, such as, for

example, a few tenths of one per cent ofmag-ne sium-sulfate, to the slurry. The addition Or anappropriat amount of an electrolyte will permit the depOsitionofa coating of the desired'thick nss without'running or drainage of the coating; The base coating should be applied to a thickness offrom-s% to 1% of an inch;

In' nany cases it' is desirable to mill the magnesite er periclase till the final slurry is through l00-meshf In other cases, however, 'it is desir able to mill only finelyenough to permit spraying, when the coating is to be applied by spraying Relatively coarse milling reduce the shrinkage ofthe coating and thus prevents cracking that might otherwise occur'on larg surface areas.

The coverin layer should be thinly applied, since an excess amount of thi layer may: 1

The feldspar is prepared for coating by milling a in Water to a slurry with the addition of a small amount of clay. In view of the thinness of the covering'layer, it is not necessary to add an elec-' trolyte. The slurr ma be milled to a fineness of through 100 mesh, though such a degree of fineness is not ordinarily necessary,

The function of the covering layeris twofold: (1) to form an impervious film over the base layer .to prevent access of oxidizing gases to the ferrous article; and (2) to slightly penetrate the base layer and formavitreous bond therewith. The magnesia baselayer has no bond strength at all aside .irom'that contributed by its small clay con- .tent, and a limitedipenetration by the'covering layer'is' desirabl'e' in order to increase the me chanical durability of the composite coating.

I claim: 7 1. A ferrous article protected against oxidation at elevated temperature by a ceramic coating comprising a relativelythick base layer of refractory material non-reactive with iron oxide selected from the group-consisting of magnesite, peri'clase, chrome-magnesite, chromite, fo'rsterite and alumina; and a relatively thin covering layer of a vitreous material slightly penetrating the base layerto form a vitreous boncltherewith and form.

ing an impervious ifilm thereov-er, said vitreous material being selected from the group consisting tected' against oxidation at elevated temperature by a ceramic coating comprising a re'lativelythick base layer or -refractory materia l non-reactive with iron oxide selected from the group consisting of magnesite, pericl-ase,"chromite' magnesite;

chrome, -';forsterite and alumina, and a relatively thin covering layer of a 'vitreous'material slightly penetrating t-he'baselayerto form a vitreous bond therewith and forming an" impervious film thereov-er gsaid vitreous material being selected from the group consisting of feldspar, ceramic glaze compositions and glass compositionscharactere ized by melting to "an impervious film' at tem? peratures of from 1000170 1300 C.

4. A steel retort in accordance with claim 13,

in which the base layer is principally of magnesit'e from 3% to {zinc-h thick and the covering ayer'is'" an impervious film of fused feldspar.

' LUTHER "D. 'FEITER'OLF." 

